because it doesn't preserve the alpha's and you loose the particular blend modes of each layer when you want to group those layers together. what's the deal?
and also if you've got some keyframe animation on some layers and then want to make a global scale animation it doesnt work. because it sets the key after the last keyframe animation you did and you cant make global keyframes (layered keys). only if you nest layers, then you can set new ones on it. but as i said above, i cant use it since i loose my blending modes. and yes, i checked on the "preserve alphas".